Positive resist terpolymer composition and method of forming resist pattern

ABSTRACT

PCT No. PCT/JP78/00021 Sec. 371 Date July 2, 1979 Sec. 102(e) Date July 7, 1979 PCT Filed Nov. 6, 1978 PCT Pub. No. WO79/00283 PCT Pub. Date May 31, 1979 
     A cross-linkable positive-working ionizing radiation-resist or ultraviolet ray-resist polymer composition comprising, in polymerized form, 
     (a) from about 70 to about 99% by mole of units derived from a methacrylic acid ester of the formula: 
     
         CH.sub.2 ═C(CH.sub.3). COOR 
    
     where R is an alkyl or haloalkyl group having from 1 to 6 carbon atoms, a benzyl group or a cyclohexyl group, 
     (b) from about 1 to about 20% by mole of units derived from methacrylamide, and 
     (c) from about 0.05 to about 20% by mole of units derived from methacrylic acid chloride; 
     each amount of the units (a), (b) and (c) being based on the total moles of the units (a), (b) and (c). The polymer composition is preferably in the form of either a copolymer comprised of the units (a), (b) and (c), or a blend of a copolymer comprised of the units (b) and a portion of the units (a) and a copolymer comprised of the units (c) and the remainder of the units (a). The resist polymer composition exhibits enhanced sensitivity as well as good thermal resistance, contrast and resolution.

DESCRIPTION BACKGROUND OF THE INVENTION

This invention relates to a cross-linkable positive-working ionizingradiation-resist or ultraviolet ray-resist polymer composition and to amethod of forming a positive resist pattern on a substrate using theresist polymer composition.

Polymethyl methacrylates have heretofore been widely used aspositive-working resists in electron or X-ray lithography. Although thepolymethyl methacrylate resists exhibit satisfactory resolution, theirsensitivity to radiation is poor. In other words, there is only a slightdifference in solubility between irradiated regions and non-irradiatedregions, particularly at low ionizing radiation exposures. Accordingly,it has eagerly been desired to provide polymeric resist materialsexhibiting a resolution approximately similar to and a sensitivity fargreater than those of the conventional polymethyl methacrylate resists.

In order to provide methyl methacrylate polymer resists of improvedsensitivity, it has been proposed to copolymerize methyl methacrylatewith a sensitivity-enhancing acrylic monomer such as hexafluorobutylmethacrylate. However, such a methyl methacrylate copolymer resist isstill unsatisfactory in that it has an undesirably low softening pointand a poor thermal resistance as compared with the conventionalpolymethyl methacrylate resists.

In U.S. Pat. No. 3,981,985, a mixture comprised of (a) a copolymer of amonoolefinically unsaturated carboxylic acid, such as methacrylic acid,and a monoolefinically unsaturated compound, such as methylmethacrylate, and (b) a copolymer of a monoolefinically unsaturatedcarboxylic acid chloride, such as methacrylic acid chloride and amonoolefinically unsaturated compound was also proposed for use as apolymer resist. The proportions of the two copolymers (a) and (b) inthis mixture are such that the carboxylic acid and the carboxylic acidchloride are essentially stoichiometric. When a resist coating of thismixture is heated, carboxylic acid anhydride cross-links are formed, andthus the resist coating becomes thermally resistant. However, thesensitivity of this resist coating is still not completely satisfactory.

SUMMARY OF THE INVENTION

The main object of the present invention is to provide positive-workingionizing radiation-resist or ultraviolet ray-resist polymeric materialswhich exhibit enhanced sensitivity as well as good thermal resistance,contrast and resolution.

The other objects and advantages of the present invention will beapparent from the following description.

One aspect of the present invention provides a cross-linkablepositive-working ionizing radiation-resist or ultraviolet ray-resistpolymer composition, comprising, in polymerized form,

(a) approximately 70 to 99% by mole of units derived from a methacrylicacid ester of the formula:

    CH.sub.2 ═C(CH.sub.3).COOR

wherein R is an alkyl or haloalkyl group having from 1 to 6 carbonatoms, a benzyl group or a cyclohexyl group,

(b) approximately 1 to 20% by mole of units derived from methacrylamide,and

(c) approximately 0.05 to 20% by mole of units derived from methacrylicacid chloride;

each amount of the units (a), (b) and (c) being based on the total molesof the units (a), (b) and (c). The unexpected advantage of the presentinvention resides primarily in the fact that its sensitivity is far moreenhanced than the sensitivity of the positive resist polymer compositiondescribed in U.S. Pat. No. 3,981,985, which contains substantiallystoichiometric amounts of monoolefinically unsaturated carboxylic acidunits and monoolefinically unsaturated carboxylic acid chloride units.

DETAILED DESCRIPTION

The positive resist polymer composition of the present invention is inthe form of either a terpolymer comprised of the methacrylic acid esterunits (a), the methacrylamide units (b) and the methacrylic acidchloride units (c), or a blend of at least two polymers, each of whichis comprised of at least one of the units (a), (b) and (c). Preferably,the polymer composition is either the terpolymer of the units (a), (b)and (c) or a blend of a copolymer comprised of the units (a) and (b)with a copolymer comprised of the units (a) and (c).

When a coating of either of the above mentioned polymer compositions isapplied to a substrate and heated, dehydrochlorination occurs betweenthe acid amide groups and the acid chloride groups, and athree-dimensional network of acid imide cross-links is formed. Thispolymer network is insoluble in a solvent used as a developer. It ispresumed that, when the polymer network is irradiated with ionizingradiation or ultraviolet rays, both the main chains and the cross-linksof the polymer network are destroyed at the irradiated regions, and thepolymers are degraded into lower molecular weight polymers. Thesedegraded, lower molecular weight polymers are soluble in a developersolvent, and, when an irradiated pattern in the polymer resist isdeveloped by using a solvent, the polymers in the irradiated regions areselectively removed so as to leave the positive resist pattern on thesubstrate.

The methacrylic acid esters used for the preparation of the positiveresist polymer composition are esters of an alkyl or haloalkyl grouphaving from 1 to 6 carbon atoms, a benzyl group and a cyclohexyl group.The methacrylic acid esters include, for example, methyl methacrylate,tert.-butyl methacrylate, iso-propyl methacrylate, hexafluorobutylmethacrylate, hexafluoroisopropyl methacrylate, cyclohexyl methacrylateand benzyl methacrylate. These methacrylic acid esters may be usedeither alone or in combination. The amount of the methacrylic acidesters may be varied in the range of from about 70 to about 99% by molebased on the total moles of all monomers used for the preparation of thepolymer composition. When the amount of the methacrylic acid esters istoo small, the sensitivity of the positive resists is poor. In contrast,when the amount of the methacrylic acid esters is too large, both thethermal resistance and solvent resistance of the polymer resists arepoor.

The amount of methacrylamide may range from about 1 to about 20% bymole, preferably from about 2 to about 10% by mole, based on the totalmoles of all monomers used for the preparation of the polymercomposition. When the amount of the methacrylamide is too small, thepositive resists have, when heated, an undesirably low degree ofcross-linking and are thus relatively soluble in a solvent developer andpoor with respect to thermal resistance and sensitivity. In contrast,when the amount of the methacrylamide is too large, the degree ofcross-linking is undesirably high and the sensitivity is quite poor.

The amount of methacrylic acid chloride may range of from about 0.05 toabout 20% by mole, preferably from about 0.3 to about 3% by mole, basedon the total moles of the monomers used for the preparation of thepolymer composition. When the amount of methacrylic acid chloride isoutside this range, the polymer resists are unsatisfactory, similar tothe case where the amount of the methacrylamide is outside theabove-mentioned range.

The polymer composition of the present invention may contain, inaddition to the above-mentioned methacrylic acid ester, methacrylamideand methacrylic acid chloride units, usually less than 50% by weight,based on the total weight of the polymer composition of units derivedfrom other monoolefinically unsaturated monomers, provided that thepolymer resists are not harmfully influenced.

The molar ration of the methacrylamide to the methacrylic acid chlorideshould preferably be within the range of from about 2/3 to about 100/3by mole in order to achieve the desired resist characteristics.

The polymer composition of the present invention, which is in the formof either a terpolymer of the above-mentioned units (a), (b) and (c), ora blend of polymers, each containing at least one of the units (a), (b)and (c), may be prepared in a conventional manner. The molecular weightof the polymer composition may range from about 30,000 to about1,000,000, preferably from about 30,000 to about 400,000, as determinedby a gel permeation chromatography procedure. The ratio of the weightaverage molecular weight to the number average molecular weight mayrange from 1/1 to 4/1, preferably from 1/1 to 2/1.

A positive resist pattern may be produced on a substrate as follows. Asolution of the polymer composition in a solvent such as, for example,2-ethoxyethyl acetate, 2-methoxyethyl acetate or cyclohexanone, iscoated on a substrate by using, for example, a spinner. Then, thecoating of the polymer composition is usually heated to a temperature of140° to 220° C. for a period of 5 to 30 minutes. The optimum temperatureand time period are approximately 200° C. and approximately 15 minutes,respectively. The polymer network resist so formed is irradiated withionizing radiation such as electron rays, X-rays or ultraviolet rays inaccordance with a desired pattern until the acid-imide cross-links andthe main chains are broken at the irradiated regions. The irradiatedresist is developed by applying thereto a developer solvent, such asmethyl isobutyl ketone, ethyl acetate or acetone so as to leave thepositive resist pattern on the substrate.

The invention will be explained in more detail by the followingillustrative examples, in which some of the characteristics of thepositive resists were determined as explained. Irradiation was carriedout in accordance with a vector scanning procedure by using an electronexposing apparatus (Cambridge Instrument EBMF-1).

Contrast "γ" was determined in accordance with the equation:

    γ=0.5/[log(D.sub.0 /D.sub.0.5)],

wherein D₀ is the electron dose in C/cm² required to reduce the initialresist thickness to zero and D₀.5 is the electron dose in C/cm² requiredto reduce the initial resist thickness to a half thereof. The initialresist thickness was 0.5 micron as measured after the coated resist wasbaked in order to form cross-links.

Sensitivity was expressed in terms of the electron dose (C/cm²) requiredto reduce the polymer resist thickness from 0.5 microns to zero, atleast one part thereof, when the irradiated polymer resist was dipped inmethyl isobutyl ketone or another liquid developer at a temperature of20° C. for one minute. The smaller the electron dose, the greater thesensitivity.

Resolution was evaluated by determining the minimum possible size ofeach line and each space during the production of a parallel line resistpattern having lines and spaces of the same size, and further bydetermining the maximum possible height to width ratio of each linearridge of the line pattern.

Thermal stability was evaluated in terms of the softening temperature,determined by using a scanning type electron microscope to observe theshape of the resist of a parallel line pattern while the resist wasgradually heated in a nitrogen atmosphere. The softening temperature isdefined as the lowest temperature at which the polymer resist loses itspredetermined shape and starts to flow. Furthermore, thermal stabilitywas evaluated in terms of the thermal decomposition temperature, whichis determined according to a thermogravimetric analysis wherein apolymer resist specimen is heated in a nitrogen atmosphere at a rate of10° C./min. The thermal decomposition temperature is defined as thetemperature at which the weight of the specimen starts to be reduced.

EXAMPLE 1

95.0% by mole of methyl methacrylate (MMA), 2.5% by mole ofmethacrylamide (MAA) and 2.5% by mole of methacrylic acid chloride(ClMA) were copolymerized by using a conventional solutionpolymerization procedure. The terpolymer obtained (M.W.=250,000) wasdissolved in 2-ethoxyethyl acetate to obtain a 9.0% by weight solution.This solution was coated on a silicon substrate using a spinner rotatingat 4,500 rpm. The thickness of this coating when dry was 0.6 microns.The coated substrate was next heated at a temperature of 200° C. for aperiod of 15 minutes. Then, the coated substrate was irradiated withelectron rays by using an electron beam accelerator at an acceleratingvoltage of 30 kV. The irradiated substrate was dipped in ethyl acetateat a temperature of 20° C. for one minute to obtain a resist of the linepattern (specimen 1).

For comparison purposes, similar positive resists were prepared from aterpolymer (M.W.=250,000, specimen 2) made from a mixture of 93.5% bymole of methyl methacrylate (MMA), 5% by mole of methacrylamide (MAA)and 1.5% by mole of methacrylic acid chloride (ClMA); a terpolymer(M.W.=250,000, specimen 3) made from a mixture of 89.5% by mole ofmethyl methacrylate (MMA), 10% by mole of methacrylamide (MAA) and 0.5%by mole of methacrylic acid chloride (ClMA); a terpolymer (M.W.=300,000,comparative specimen 1) made from a mixture of 95.0% by mole of methylmethacrylate (MMA), 2.5% by mole of methacrylic acid (MA) and 2.5% bymole of methacrylic acid chloride (ClMA); and a conventional polymethylmethacrylate (PMMA, M.W. 300,000, comparative specimen 2). Theprocedures used in the preparation of these positive resists weresimilar to those mentioned above with respect to the specimen of thepresent invention, except that the polymethyl methacrylate resist(comparative specimen 2) was developed at a temperature of 20° C. forone minute by using a methyl isobutyl ketone/isopropyl alcohol mixturehaving a volume ratio of 1/3.

The characteristics of the positive polymer resists are shown in TableI, below.

                  TABLE I                                                         ______________________________________                                        Specimen      1      2      3    Com. 1  Com. 2                               ______________________________________                                        Composition MMA                                                                             95.0   93.5   89.5      95.0 100                                (mole %) MAA  2.5    5.0    10.0 MA   2.5   0                                    ClMA       2.5    1.5     0.5      2.5   0                                 ______________________________________                                        Contrast (γ)                                                                          5.8    5.2     4.3      4.0  3.1                                Sensitivity (C/cm.sup.2)                                                                    2 ×                                                                            1 ×                                                                            6 × 5×                                                                           1.6 ×                                      10.sup.-5                                                                            10.sup.-5                                                                            10.sup.-6 10.sup.-5                                                                          10.sup.-4                          Resolution                                                                    min. size of line and                                                                       0.2    0.2    0.2       0.2  0.2                                space (μ)                                                                  max. ratio of H/W                                                                           7      6      6         6    2.5                                Thermal stability                                                             Softening temp. (°C.)                                                                145    150    155       140  110                                Decomposition temp.                                                                         320    320    330       300  250                                (°C.)                                                                  ______________________________________                                    

EXAMPLE 2

Following a procedure similar to that mentioned in Example 1, a polymerresist pattern was produced wherein the following copolymer blend wasused instead of the MMA/MAA/ClMA terpolymer. The copolymer blend usedwas comprised of 50% by weight of a copolymer (M.W.=250,000) of 92.0% bymole of methyl methacrylate (MMA) and 8.0% by mole of methacrylamide(MAA) and 50% by weight of a copolymer (M.W.=180,000) of 98.8% by moleof methyl methacrylate (MMA) and 1.2% by mole of methacrylic acidchloride (ClMA).

The characteristics of the polymer resist pattern are shown in Table II,below.

EXAMPLE 3

Following a procedure similar to that mentioned in Example 1, a polymerresist pattern was produced wherein the following terpolymer was usedinstead of the MMA/MAA/ClMA terpolymer. The terpolymer used was preparedby using a conventional solution polymerization procedure and wascomprised of 95.0% by mole of benzyl methacrylate (BzMA), 4.0% by moleof methacrylamide (MAA) and 1.0% by mole of methacrylic acid chloride(ClMA). The terpolymer had a M.W. of approximately 200,000.

The characteristics of the polymer resist pattern are shown in Table II,below.

                  TABLE II                                                        ______________________________________                                        Specimen       Ex. 2        Ex. 3                                             ______________________________________                                        Composition    MMA      46.0    BZMA   95.0                                   (mole %)       MMA       4.0    MAA    4.0                                                   MMA      49.4    ClMA   1.0                                                   ClMA      0.6                                                  ______________________________________                                        Contrast (γ)                                                                           4.3          3.5                                               Sensitivity (C/cm.sup.2)                                                                     6 × 10.sup.-6                                                                        8 × 10.sup.-6                               Resolution                                                                    min. size of lin                                                                             0.2          0.3                                               space (μ)                                                                  max. ratio of H/W                                                                            6            5                                                 Thermal stability                                                             Softening temp. (°C.)                                                                 150          135                                               Decomposition temp. (°C.)                                                             320          300                                               ______________________________________                                    

What is claimed is:
 1. A cross-linkable positive working resist polymercomposition comprising, a terpolymer comprised of(a) from about 70 toabout 99% by mole of units derived from a methacrylic acid ester of theformula:

    CH.sub.2 ═C(CH.sub.3).COOR

where R is selected from the group consisting of alkyls having from 1 to6 carbon atoms, haloalkyls having from 1 to 6 carbon atoms, benzyl andcyclohexyl, (b) from about 1 to about 20% by mole of units derived frommethacrylamide, and (c) from about 0.05 to about 20% by mole of unitsderived from methacrylic acid chloride;each amount of the units (a), (b)and (c) being based on the total moles of the units (a), (b) and (c). 2.A polymer composition according to claim 1, wherein the molar ratio ofthe units (b) to the units (c) ranges from about 2/3 to about 100/3. 3.A polymer composition according to claim 1 or 2, wherein the amount ofthe units (b) ranges from about 2 to about 10% by mole based on thetotal moles of the units (a), (b) and (c).
 4. A polymer compositionaccording to claim 1 or 2, wherein the amount of the units (c) rangesfrom about 0.3 to about 3% by mole based on the total moles of the units(a), (b) and (c).
 5. A polymer composition according to claim 1 or 2,wherein the methacrylic acid ester is comprised of at least one monomerselected from the group consisting of methyl methacrylate, tert. butylmethacrylate, cyclohexyl methacrylate and benzyl methacrylate.
 6. In amethod of forming a positive resist pattern on a substrate, of the typeincluding the steps of(i) coating the substrate with a solution of across-linkable positive-working polymeric material, (ii) heating thecoated substrate to form cross-links in the polymer resist coating,(iii) irradiating the cross-linked polymer resist with ionizingradiation or ultraviolet rays in accordance with a desired pattern, andthen (iv) applying a solvent to the irradiated polymer resist thereby toleave the desired positive resist pattern on the substrate,theimprovement comprising: using as the cross-linkable positive-workingpolymeric material in step (i) a polymer composition comprising, aterpolymer comprised of (a) from about 70 to about 99% by mole of unitsderived from a methacrylic acid ester of the formula:

    CH.sub.2 ═C(CH.sub.3).COOR

wherein R is selected from the group consisting of alkyls having from 1to 6 carbon atoms, haloalkyls having from 1 to 6 carbon atoms, benzyland cyclohexyl, (b) from about 1 to about 20% by mole of units derivedfrom methacrylamide, and (c) from about 0.05 to about 20% by mole ofunits derived from methacrylic acid chloride;each amount of the units(a), (b) and (c) being based on the total moles of the units (a), (b)and (c).
 7. A method according to claim 6, further comprising using amolar ratio of the units (b) to the units (c) in the range of from about2/3 to about 100/3.